1. Field of the Invention
The present invention relates to a process for preparing a lipid composition having a high .gamma.-linolenic acid content and, more particularly, to a process for preparing such a composition from a specific living microorganism.
2. Description of the Prior Art
6,9,12-octadecatrienoic acid or .gamma.-linolenic acid (to be referred to as GLA hereinafter) is a fatty acid synthesized in a living organism from linoleic acid. GLA is converted into prostaglandin E.sub.1, F.sub.2, E.sub.2 or F.sub.2 through bishomo-.gamma.-linolenic acid. It has been found recently that the in vivo conversion reaction of linoleic acid into GLA is hindered by aging, alcohol drinking, and vitamin deficiency. An imbalance in prostaglandin due to a GLA deficiency is considered to be a factor causing allergic diseases, thrombosis, or cancer.
GLA which is therefore important to the health of living organisms is obtainable from plant seeds such as seeds of the evening primrose. However, GLA is contained in evening primrose seeds in small amounts and accounts for at most 10 % by weight of the total fatty acid content. Furthermore, plant seed oil also contains about 70 % by weight based on the total fatty acid content of linoleic acid. When GLA is obtained by refining a fatty acid mixture obtained from plant seed oil by solvent fractionation or the like, GLA cannot be easily separated from linoleic acid since the two components behave in a similar manner.
It has been proposed to obtain GLA from the lipids of microorganisms. See, for example, R.O. Mumma, Lipids, 6, 584 (1971); R. Shaw, Biochem. Biophys. Acta. 98, 230 (1965); and Suzuki et al., Yukagaku (Oil Chemistry), 30, 863, (1981). However, the GLA content of the GLA producing microorganisms mentioned in these articles is low and represents at most 10 to 20 % of the total lipid content. Japanese Patent Publication (Kokoku) No. 58-22199 reports that when mold fungi of the genus Mortierella are cultured in a medium to which a hydrocarbon is added, the cultured fungi have a GLA content of 20 % or more based on the total fatty acid content. However, mold fungi of the genus Mortierella grow at a slow rate and grow particularly slowly at a low temperature which is most conducive for them to produce GLA. Therefore, the GLA productivity of these fungi is low.
The present invention is directed to a technique for producing GLA from cultured microorganisms.